1. Field of the Invention
The invention relates to a drive train, particularly a vehicle drive train, which has a hydrodynamic machine and a cooling medium circuit, the cooling medium being at the same time the working medium of the hydrodynamic machine.
2. Description of Related Art
Drive trains, particularly motor vehicle drive trains having hydrodynamic machines are known in a plurality of designs. For example, hydrodynamic brakes, so-called retarders, are used nowadays in heavy-duty motor vehicles for wear-free braking of the vehicle.
Such hydrodynamic machines, particularly retarders, have a working chamber, which can be filled with a working medium in order to transmit torque or driving power, respectively, without any wear, from a primary impeller to a secondary impeller of the hydrodynamic machine—for example, from the rotor to the stator of the retarder. Corresponding to the desired power transmission, a specific quantity of working medium is conducted into the working chamber of the hydrodynamic machine. The carrying of the working medium into the hydrodynamic machine, out of it, and/or within the hydrodynamic machine is controlled by at least one control valve and, as a rule, by several control valves. The one or more control valve(s), which are designed, for example, as solenoid valves, may, in turn, be actuatable by at least one control unit associated therewith—for example, by a flow of current through the magnet coil in the respective solenoid. The control valves are incorporated in a control air system or compressed air system, for example, and control the compressed air state in specific sections, particularly line sections, of the control air system. The air from the control air system controls regulating or shutoff valves and/or collars and/or flaps in the working medium circuit of the hydrodynamic machine—for example, an inlet valve and/or an outlet valve for a retarder or, in general, a hydrodynamic machine. Accordingly, air (or also another control medium) flows through the control valves, in contrast to the valves, collars, or flaps in the working medium circuit, which are switched by the control air.
The control unit comprises electronic components, which are subjected to heating during power operation. In order to be able to fabricate the control unit at an economically practical cost from comparatively inexpensive materials, the maximum allowable temperature of the control unit is limited. The temperature of the control unit is then a function of the ambient temperature as well as the extent of self-warming. Nowadays, for example, it is usual in the motor vehicle sector to limit the maximum ambient temperature in the engine compartment in which the control unit is disposed to 120° C., so that an inadmissible heating of the electronic components of the control unit is prevented.
In recent times, however, vehicle manufacturers have striven to allow higher ambient temperatures in the vehicle—for example, a temperature range of 40° C. (under Arctic conditions, from −50° C.) to over 120° C. Because of this, the higher continuously allowed ambient temperatures result in limitations in the allowable maximum period of time that the electronic control unit of hydrodynamic machines may be turned on as well as, in particular, the control valves thereof, which are designed as solenoid valves, because the allowable temperature range for self-heating is correspondingly reduced by the difference of the higher ambient temperature with respect to the previously maximum allowable ambient temperature. Due to the limitation of the maximum allowable period of time for operation over a pre-given time interval, the self-heating is reduced and accordingly the maximum allowable temperature of the materials of the electronic control unit or of the control valves and/or the surroundings thereof is maintained.
Such limitation of the operating time, for example, by a time-dependent switching on of the control unit and/or by a time-dependent actuation of the valves by way of an advance setting of defined time pauses between individual actuations or by way of pulse-width modulation, has a detrimental effect on the availability of the hydrodynamic machine and is accordingly undesirable.